We’re all going to be hearing a lot about intracranial pressure over the next hours and days, so I thought it would be useful to understand a bit about it. Here we go.
The brain is enclosed in a rigid closed box. Since the volume of the box is fixed in adults, the addition of any extra “stuff” won’t take up more space, it’ll increase the pressure. What kind of “stuff”? Well in this case, at least yesterday, the extra stuff was a blood clot. This clot now has crammed itself into the same snug volume as the brain, kinda like when your kids jump into bed with you. The pressure goes up. Treatment is obvious – take out the clot!
Another “extra” element which can try to stuff itself into the skull is the brain itself. Remember that what we call the brain actuall contains three things: brain tissue, cerebrospinal fluid (or CSF), and the blood volume contained in the arteries, veins and capillaries of the brain. An abnormal increase in the volume of any of these “compartments” can, if big enough, lead to increased pressure. In Michael’s case, what we don’t want to have happen is to see the brain tissue itself swell. This would mean the brain cells themselves are not maintaining their volume, implying a severe level of injury. This kind of edema (swelling) is also harder to treat. Here, one attempts to optimise the flow of CSF and blood return to the heart by nursing the patient 30° head up. The brain needs to be properly perfumed with blood containing the right amounts of oxygen and glucose to maintain healthy cellular metabolism. If the pressure is much too high, the patient will be deeply sedated in order to protect the brain by decreasing its metabolic requirements. Lastly, the neurosurgeon can remove a large flap of skull bone. The “box” is now open, the pressure is now normal, and the swelling now expresses itself as a brain that is actually swollen without many of the deadly side effects of elevated ICP. This extreme measure, regularly carried out with severe head injury, is called a decompressive craniectomy.
Why is elevated intracranial pressure (ICP) bad?
There are at least 3 reasons. First is that elevated ICP means something bad has happened to the brain. This primary injury, whatever it is, has already done damage and that’s of concern. Second, this pressure acts to compress the blood vessels nourishing the brain. The higher the ICP, the worse the blood flow. So there’s a vicious circle: the ICP goes up, blood flow down. The cells suffer from lowered oxygen delivery and start to swell, making the ICP worse. Bad news. The third reason is mechanical. The brain is held in place in the skull by tough sheaths, between the hemispheres, and between the brain and the cerebellum. When the ICP goes much too high, it can actually force the brain to slide under one or the other of these partitions, usually with catastrophic consequences.
Hope this helps!